1. Field of the Invention
The present invention relates to a peer to peer (P2P) content delivery system which includes a plurality of node devices which are connected with each other via a network and, especially, relates to a technical field of an information delivery system, delivery request program, transfer program, delivery program, and the like in which content data which is to be delivered from one node device to the other is distributed and saved in a plurality of node devices.
2. Discussion of Related Art
In this kind of content delivery system, each of node devices has a list in which location information regarding content data distributed and saved in a plurality of node devices is described (for example, information which enables to specify an IP address or the like of a node device which saves the content data by designating auxiliary information such as a title of the content data: Hereinafter referred to as index information) and on the basis of index information described in the list, it becomes possible to download content data which a user wants. Such a list is common information to be used commonly among a plurality of node devices and generally is managed by a management server which manages all the content data saved on a content delivery system so that the list is delivered from the management server to each node device.
For example, in the Patent Document 1, as this kind of a management server, an index server which is most magnificent and manages all the content information in a content delivery system has been disclosed.
Moreover, in the Patent Document 2, as a method which does not use a management server, a technique using a distributed hash table (DHT) has been disclosed. In this system, a request node device which requests delivery of content inquires location of a node device, which saves content information, to a management server which manages content information to access the node device.
Furthermore, as disclosed in the Patent Document 3, pure P2P delivery system such as Gnutelia, Freenet, and Winny has been invented. However, in these systems, identification information regarding content is designated to search for the content, specify location thereof, and access thereto. In such a system, terminals communicate with each other to search and transfer the content.    Patent Document 1: Japanese Published Unexamined Patent Application No. 2002-318720    Patent Document 2: Japanese Published Unexamined Patent Application No. 2004-127189    Patent Document 3: Japanese Published Unexamined Patent Application No. 2004-185263
Meanwhile, in a peer to peer content delivery system, a node device withdraws (due to power cut-off to the node device, or partial disconnection on a network) and participates in the system with a high frequency. In addition, content data is newly saved or deleted to or from the node device with high frequency. Therefore, the above-mentioned list must be updated often. Hence, to keep the list to the latest condition, it is regarded that a management server, one that was described above, is required.
However, in a content delivery system in which a management server manages the list, the more the number of node device is increased, the more server load is increased when updating the list and, at the same time, there arises a problem that network load is concentrated on one spot, restricting even a list which can be delivered from being delivered. Therefore, this system is not preferable. Moreover, when the management server is stopped (for example, due to power outage or malfunction), the list cannot be updated. Furthermore, there is also a problem of management cost for managing a list with a server.
In addition, in a method that does not use a management server, since a content node device which retains content is not always working, when a management node device which was inquired about location of a content node device, which saves content requested to be delivered, from a request node notifies the location of the content node device based on information recorded in the list of the management node device, the request node finds upon requesting delivery of the content that the content node has already withdrawn, has been stopped (for example, due to power outage or malfunction), or the desired content has already been deleted. In such a case, the request node must search for location of a content node device from the beginning. If this occurs frequently, in a system that contains vast quantity of content or a system in which a large number of devices participate in, large amount of communication is performed between each device, causing great load to the whole of the system.